Method of producing alkali alcoholates

ABSTRACT

The present invention is directed to a method for the electrochemical production of alkali alcoholates and to an electrolytic cell that can be used for this purpose. In particular, the invention is concerned with a method of production in which aqueous saline solutions are used on the anode side. A main characteristic of the electrolytic cell is that the anode compartment and the cathode compartment are separated by an anolyte-stable and catholyte-stable solid electrolyte.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to German application DE 101 54 082.5, filed on Nov. 2, 2001, the contents of which is hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention is directed to a method for the electrochemical production of alkali alcoholates and to an electrolytic cell that can be used for this purpose. In particular, the invention concerns a method of production in which aqueous saline solutions can be used on the anode side.

BACKGROUND OF THE INVENTION

[0003] Alkali alcoholates (Na/KOMe/OEt, etc.) are among the standard products of the chemical industry. Compounds in which the alcohol component contains up to 4 carbon atoms are especially valuable in organic synthesis.

[0004] Several methods are known for the synthesis of alkali alcoholates (F. A. Dickes, Berichte der Deutschen Chemischen Gesellschaft 63:2753(1930)). One such method involves the dewatering of solutions containing the alcohol and the corresponding alkali hydroxide in equivalent amounts. The displacement of equilibrium toward the desired product can take place thereby by distillation measures or with the aid of semipermeable membranes (EP 0 299 577).

[0005] The alcoholate can also be obtained by dissolving an alkali metal in the corresponding alcohol. However, sodium and potassium react so violently with lower alcohols that usage on an industrial scale would be too risky and higher or branched-chain alcohols may react too slowly. DE 19802013 suggests a solution to this problem. Specifically, amalgam stemming from chlor-alkali electrolysis is reacted with alcohol under the catalytic action of transition metal nitrides and transition metal carbides. This method has the disadvantage that the alcohols can be obtained more or less only contaminated with mercury which must then be removed in additional steps.

[0006] DE 3346131 suggests an electrochemical method in which alkali-metal ions are produced from alcoholic saline solutions on the anode side using a cation exchange membrane as separator material. Alkali-metal ions migrate via the membrane into the cathode compartment, where alcohols are transformed into their anions during the development of hydrogen.

[0007] U.S. Pat. No. 5,425,856 discloses a continuous electrochemical method for the production of alkali alcoholates starting from alcoholic saline solutions. This method has the disadvantage that chlorinated alcohol compounds arise in the anode compartment as by-products and these compounds may be pathogenic.

[0008] DE 19603181 also suggests an electrochemical method for the production of alkali alcoholates. In this procedure, the anode compartment and the cathode compartment are totally separated by an ion-conducting solid electrolyte in order to avoid a disadvantageous mixing of the anolyte and catholyte solutions.

[0009] All of the electrochemical methods discussed above operate entirely in alcoholic solution. The conductivity of such systems is limited and, as a result, a low electrode spacing must be used or a conducting salt must be added to the solution to raise the conversion rate to a tolerable level.

[0010] PCT/EP00/08278 presents another method for the production of alkali metals. An aqueous solution of an alkali-metal salt is placed in the anode compartment and this compartment is separated helium-tight from the cathode compartment containing, e.g., a steel cathode. A saline melt is suggested as a cathode-side liquid electrolyte. In this method, a mixing of anolyte and catholyte must be avoided under all circumstances. Therefore, a solid electrolyte is suggested as separator that conducts only the considered cations and which totally suppresses the diffusion of water into the cathode compartment. Exclusively coated, ion-conducting solid electrolytes are suggested as separators resistant to aqueous systems.

SUMMARY OF THE INVENTION

[0011] The present invention concerns an electrochemical method for the production of alkali alcoholates that avoids many of the disadvantages of other procedures. The method is suitable for use on an industrial scale both from an economic and an ecological standpoint. More specifically, the method may be used for the cost-effective preparation of the desired products while avoiding the emission of pathogenic substances.

[0012] In its first aspect, the invention is directed to a method for the electrochemical production of alkali alcoholates starting from a saline, aqueous anolyte and an alcoholic catholyte. The cathode compartment and the anode compartment of the electrolytic cell are separated by a solid electrolyte that conducts only ions and is anolyte-stable and catholyte-stable. Preferably, β-aluminum oxide is used as solid electrolyte and the solid electrolyte is coated on one side or on both sides with one or several ion-permeable, electrically conductive layers. These layers may be applied to the solid electrolyte by vapor deposition or they may be burned onto its surface as an immersion layer. Within the framework of this invention the expression “ion-permeable” is to be understood to signify that ions can diffuse through the material on account of porosities and not on account of defects (as in ionic conduction) in this material.

[0013] Typically, the cathode side and the anode side surfaces of the solid electrolyte will be in contact with the means supplying and removing current. In a preferred embodiment, the solid electrolyte is the form of a plane-parallel plate, i.e., a plate with parallel faces. In an alternative embodiment, the anode compartment and/or the cathode compartment of the electrolytic cell is/are filled with electrically conductive powders.

[0014] In general, the temperature of the anolyte during the electrolysis should be between −10° C. and 0° C., and the temperature of the catholyte should be between −45° C. and −5° C., calculated in each instance from the boiling point of the anolyte or of the catholyte. Preferably the method is carried out in a continuous manner with the anolyte at a pH of>7.

[0015] In another embodiment, the invention is directed to an electrolytic cell that comprises an anode compartment containing an aqueous solution of at least one alkali-metal salt as anolyte, and a cathode compartment. The cell has a solid electrolyte as membrane by means of which the anode compartment and the cathode compartment are separated from one another. In one embodiment, the solid electrolyte does not comprise any other ion-conducting layer and the solid electrolyte is, preferably, β-aluminum oxide. Alternatively, the solid electrolyte may be coated on one side or on both sides with one or several ion-permeable, electrically conductive layers. These can be applied to the solid electrolyte by vapor deposition or they can be burned onto its surface as immersion layer. The cathode side and the anode side surfaces of the solid electrolyte will usually be in contact with the means supplying and removing current. In a preferred embodiment, the anolyte has a pH of>7.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The invention is directed to a method for the electrochemical production of alkali alcoholates starting from a saline, aqueous anolyte and an alcoholic catholyte. In this method, the cathode compartment and the anode compartment are separated by a solid electrolyte that conducts only ions and is anolyte-stable and catholyte-stable. The method may be used to manufacture water-sensitive alkali alcoholates from aqueous saline solutions and simple alcohols that are readily available using electric current without entailing the danger of a contamination of the products with mercury. In addition, due to the aqueous anolyte solution that is a good conductor of the electric current, the voltage used can be held lower than in methods previously described. This permits a more economical production of the alkali alcoholates. In order to further increase the conductivity of the cell, a conducting salt such as, e.g., the alkali alcoholate to be produced, can also be added to the catholyte on the order of<5% by wt., preferably<4% by wt., more preferably<3% by wt. and especially preferably<2% by wt.

[0017] In principle, all ion-conduction solid electrolytes can be used in so far as they meet the prerequisites indicated above. Suitable solid electrolytes should, in addition, have the most selective conductivity possible for the specific alkali ion with the lowest possible specific resistance. Such solid electrolytes are mentioned, for example, in GB 1155927. A solid electrolyte of the β-aluminum oxide type is preferably used. The solid electrolyte has the approximate composition Alk₂O₁₁Al₂O₃ in which Alk preferably stands for sodium, potassium or lithium as a function of which alkali alcoholate is to be produced. The solid electrolyte can optionally also be provided with the ion-conducting coatings described in PCT/EP00/08278.

[0018] In one preferred embodiment, the solid electrolyte is coated on one or both sides with one or several ion-permeable, electrically conductive layers. The layer, or layers, are preferably applied to the solid electrolyte by vapor deposition or burned on to its surface as an immersion layer. Production methods for solid electrolytes coated in this manner are taught in the literature (Edelmetall-Taschenbuch, 2^(nd) edition, Huthig-Verlag Heidelberg; Thick Film Technology: A User's Guide. London, Andy. Cermalloy Div., Heraeus Inc., West Conshohocken, Pa., USA. Editor(s): N. H. Kordsmeier, Jr.; Charles A. Harper, et al., Electron. Mater. Processes, Int. SAMPE Electron. Conf., 1^(st) Materials Science of Thick Film echnology(1987)); R. W. Vest, Am. Ceram. Soc. Bull. 65(4):631-6 (1986)). The stability of the solid electrolyte against the anolyte solution and the catholyte solution can be further increased by occasionally burning on the primer coating at>1200° C. Again, a preferred embodiment is one in which the cathode side and the anode side surfaces of the solid electrolyte coated with an electrically conductive, porous layer are in contact with the means supplying and removing current. This embodiment has the advantage that only the solid electrolyte is located between the primary anode and cathode, and, as a result, the transfer resistance, which can be characterized as a voltage drop of the cell, can be substantially minimized.

[0019] The solid electrolyte can assume any form known in the art, such as casings, disks or tubes. However, a solid electrolyte in the form of a plane-parallel plate, preferably a disk, is preferred because of its two freely accessible surfaces.

[0020] Another embodiment that can be used is one in which the anode compartment and/or cathode compartment is/are filled with electrically conductive powders in order to improve the distribution of potential in the catholyte and anolyte. The electrode surface is enlarged with this measure and, as a result, the overvoltage phenomena, especially due to diffusion problems, is reduced as much as possible. Potential electrically conductive powders are preferably those used for the decomposition of amalgam in alcohols (DE19802013) or for the anode coating in electrolyses (De Nora, et al., Chem. Ing. Tech. 47:125-128 (1975); Electrochemical Engineering: Science and Technology in Chemical and Other Industries, H. Wendt; G. Kreysa (1999), 277, pp. 265 ff; A. Mottram Couper, Derek Pletcher; Frank C. Walsh, Chem. Rev. 90(5):837-65 (1990)). These include metal powders of every type in the cathode compartment provided that they are sufficiently stable and do not result in a contamination of the alkali alcoholates. Included among preferred metal powders are nickle powders, tungsten powders, titanium powders, copper powders and steel powders. The use of Hastelloy powders is especially preferred. Another powder that can be used is graphite, either alone or in combination with one or more metal powders or modified in accordance with DE198020123.

[0021] The following may be used in the anode compartment: graphite powder, titanium powder and materials that are used for dimensionally stable anodes (Ullmann's Encyclopedia of Industrial Chemistry, vol. A6, pp. 450-454, VCH Weinheim, 1996; D. L. Caldwell, in Comprehensive Treatise of Electrochemistry, vol. 12, Plenum 1981 ed. Bockris, pp. 122-126; Comninellis, et al., J. Appl. Electrochem. (1991), 21(4):335-345 (1991); Hinden, et al., Eur. Pat. Appl. 91982)). Graphite is preferably used.

[0022] The alcohol that forms the alkali alcoholate is used as catholyte in the electrolysis. There is greater freedom with regard to saline solutions to be used as anolyte. The concentration of the saline solution should be as high as possible. Therefore, saturated solutions of carbonates, chlorides, sulfates, sulfites, hydroxides, etc. are preferably used. The use of carbonates, sulfates and chlorides is especially preferred. The saline anolyte solution should have a pH at which the solid electrolyte has its maximum stability. Since the solid electrolytes are preferably oxides of aluminum that have a rather high chemical stability in the basic pH range, the anolyte should be adjusted with special preference to pH's greater than 7 and with even greater preference to pH's greater than 8.

[0023] The conversion achieved using the methods discussed above is temperature-dependent. The higher the temperature in the electrolysis, the higher the current flow at a given voltage. The temperature should therefore be held as high as possible for reasons of efficiency. However, boiling of the anolyte solutions and catholyte solutions should be avoided from an engineering standpoint. Nevertheless, work can be performed above the boiling point if solutions are put under pressure, the pressure resistance of the solid electrolyte setting a natural limit. The temperature of the anolyte during the electrolysis is preferably adjusted to a range from greater than −10° C., preferably greater than −5° C., to less than 0° C., and preferably less than −1° C., calculated from the boiling point of the anolyte. Likewise, the temperature of the catholyte during the electrolysis is adjusted to a range from greater than −45° C., preferably greater than −20° C., to less than −5° C., and preferably less than −10° C., calculated from the boiling point of the catholyte. In a batch method (no circulation of anolyte and catholyte) the temperature of the catholyte is approximately 1-2 degrees higher than the anolyte.

[0024] It is possible to make the method continuous, especially by arranging a disk as solid electrolyte with two electrolyte compartments comprising an inlet and an outlet. A disk with 70 mm diameter is fixed between two half shells via an appropriate sealing system. The system should seal both hollow compartments (anolyte compartment and catholyte compartment) in an airtight manner and thus prevent a passage of the electrolyte solutions into one another. Electrolyte can flow through the electrolyte compartments (approximately 25 ml) in a defined manner via hose pumps. The volumetric flow rate is advantageously selected for the aqueous anolyte sols in a range of 0.11 l/h to 10 l/h, preferably 1 l/h to 2 l/h. In order to carry out a purposeful temperature adjustment, the anolyte can be heated via a thermostat. If the anolyte compartment is filled with powder, a separator system (filter) should be arranged in front of the outlet line. The catholyte solution is also pumped in this procedure through the catholyte compartment and optionally separated from entrained powder by filter insert before leaving the catholyte compartment. The described arrangement makes it possible to heat the anolyte substantially above the boiling temperature of the catholyte alcohol system.

[0025] In order to prevent the boiling of the alcohol system, there should not be a continuous heating of the catholyte and a cooling system can optionally be provided. The catholyte should be moved into the temperature range indicated above only at the beginning of the test in order to rapidly obtain the desired reaction rate. To further prevent a boiling of the alcohol, its flowthrough rate can be appropriately raised in an advantageous manner. The volumetric flow rate in this instance can be between 1 and 10 l/h, and preferably in a range of 2 to 5 l/h. The continuous circulation brings about a removal of the gaseous products from the electrolyte compartments. In order to prevent a pressure buildup in the total system, the intermediate containers used as pump receiver and for thermostating can be equipped with nitrogen-veiled reflux condensers open at the top.

[0026] The anode materials to be used in the method are well known in the art. For example, the materials cited in PCT/EP00/08278 can be used for this purpose. Materials discussed above for filling the anode compartment are preferably also used as anode material. The corresponding situation applies to the cathode material.

[0027] In another embodiment, the invention concerns an electrolytic cell that comprises an anode compartment containing an aqueous solution of at least one alkali-metal salt as anolyte, a cathode compartment and a solid electrolyte as membrane by means of which the anode compartment and the cathode compartment are separated from one another, and in which the solid electrolyte does not comprise any other ion-conducting layer. It was surprisingly determined that solid electrolytes of the type cited in PCT/EP00/08278 and not coated with ion-conducting layers have sufficient stability against the solutions used so that they can be employed on an industrial scale.

EXAMPLES

[0028] Determination of the Water Stability

[0029] An arrangement of the solid electrolyte is designed in such a manner that an ageing (age-hardening) of a casing (tube open at the top) in aqueous solution is possible. For this, the inner compartment is tightly closed by a seal and a glass headpiece. The glass headpiece is provided with an escape valve for pressure compensation and for taking specimens. The casing is fixed in a thermostatable container filled with a saturated soda solution. The casing itself is filled with 30% sodium methylate solution. The ageing test is carried out at a temperature of 60° C. The resistance of the solid electrolyte is checked by a weekly determination of the water content in the alcoholate solution. After 3 months the test can be halted and there should not be an elevation in the water content in the alcoholate solution.

[0030] In a second test, a disk (d 70 mm) is fixed between two half shells, one filled with saturated soda solution and other with 30% alcoholate solution. The half shells are fastened in a tightly sealed manner against the disk and the ambient. The construction is thermostated in a water bath at 60° C. After 3 months there should be no water in the alcoholate solution.

[0031] Preliminary Test

[0032] Part 1:

[0033] A tube, open on one side (Ionotec Ltd., B1-100-LNZ) with the dimensions 100×30×1.3 (hxDxd) was filled with methanol and, in a second test, with 10% sodium methylate solution. The tube was tightly closed and aged in sodium carbonate solution at a temperature of 50° C. After 4 weeks, in each instance the water component of the alcohol solution was determined. The analyses yielded values below 2%, which is a reflection of the tightness of the Na-beta-A1203 membrane.

[0034] Part 2:

[0035] A disk (Ionotec Ltd., D65-2-LNZ) with the dimensions 65×1 (Dxd) was fixed between two half shells designated as the anolyte compartment and the catholyte compartment. The anolyte compartment was filled with saturated sodium carbonate solution and the catholyte compartment with 10% sodium methylate solution. The tightly closed system was tempered in a water bath at 55° C. A total of 7 specimens were drawn out of the catholyte compartment at intervals of 4 weeks and analyzed for their water content. The amount of alcoholate solution removed was filled back in each instance. The analyses indicated no rise of the water content in the catholyte for the entire testing time.

Example 1

[0036] The aged tube from preliminary test 1 was used for electrochemical syntheses. Graphite felt was placed on the outer surface for contacting and connected via platinum wires to the anode output of a rectifier. The inner compartment was filled with graphite powder (particle size<50 μm) up to 20 mm below the edge and also connected to the rectifier via a platinum wire. The inner compartment was filled with 40 ml pure methanol and adjusted via a thermal element to 59° C. The tube discharge was provided with a water cooler and the anolyte was tempered to 65° C. Using this system, a static current-voltage curve was recorded (0.5 V/5 min). The data is presented in table 1.

Example 2

[0037] The tube used in example 1 was cleaned and filled with Hastelloy powder (Praxair, NI-544). A test was then performed analogously to example 1.

Example 3

[0038] The aged disk of preliminary test part 2 was also used for electrochemical syntheses. The disk was clamped between two half disks and the anolyte compartment designed so that it its temperature could be controlled. The anolyte compartment and the catholyte compartment were filled with Hastelloy powder (NI-544) and contacted via platinum wires. The degree of filling of the cell with Hastelloy powder corresponded on both sides to 98% and resulted in an available surface of 2083 mm². Current-voltage was recorded as described in example 1. The anolyte compartment was filled with saturated sodium carbonate solution and heated to 100° C. and the temperature in the catholyte compartment (3.5% sodium methylate solution) was held at 55° C.

Example 4

[0039] Instead of an untreated disk, a disk coated on both sides with Ni was used. The coating of the disk took place with a pulsed spray method (airbrush) using an Ni-resinate-toluene solution with a 1.3% nickel component at temperatures of 100° C. and a 30 minute drying phase at 300° C. The test was carried out as in example 3 except that the available surface of the separator was assumed as 100% since the nickeled surface includes the entire surface.

Example 5

[0040] Instead of a Ni-coated disk a Pt-coated disk was used. The coating took place as described in example 4. A Pt-resinate-toluene solution with 1.5% platinum component served as initial solution. The test was carried out analogously to example 4 and evaluated.

Compilation of Results from Examples 1-5

[0041] TABLE 1 Results from Examples 1-5 Voltage Time in difference Current density in A/m² minutes in V Example 1 Example 2 Example 3 Example 4 Example 5 5 0.5 0 0 1.0 1.0 0.9 10 1 0.4 0.7 2.3 2.5 2.4 15 1.5 0.6 1.3 3.8 4.2 4.1 20 2 1.1 1.9 8.4 10.1 10.2 25 2.5 1.7 2.8 14.5 19.3 15.3 30 3 2.2 3.4 20.4 25.7 24.6 35 3.5 3.1 3.4 24.8 30.8 30.7 40 4 3.4 3.4 32.3 38.6 35.6 45 4.5 3.6 3.7 27.3 45.7 43.8 50 5 4.0 4.0 32.2 52.6 47.2 55 5.5 5.1 4.0 35.6 55.8 51.6 60 6 6.7 4.2 37.5 58.1 53.4 65 6.5 7.6 4.9 36.9 60.2 54.6 70 7 8.3 5.4 36.4 62.3 54.3 75 7.5 8.9 5.8 35.5 65.7 55.7 80 8 10.1 6.3 31.9 66.9 55.9 85 8.5 10.7 6.5 30.0 67.9 56.1 90 9 11.6 6.9 26.1 68.4 56.4 95 9.5 12.7 7.2 25.0 69.1 56.8 100 10 14.0 7.2 24.3 70.0 56.3

Example 6a

[0042] The arrangement of test 3 was used except that a voltage difference of 6 V was immediately applied and not changed for 4 hours. The current density dropped from 40.8 A/m² (t=0 h) to 7.5 A/m² (t=4 h). A determination of the alcohol component in the anolyte indicated a rise of 1.2% in 20.8 g anolyte. The water component of the solution (<0.2%) remained constant.

Example 6b

[0043] The arrangement of test 4 was used. A voltage difference of 6 V was immediately applied and not changed for 4 hours. The current density dropped from 61.2 A/m² (t=0 h) to 8.6 A/m² (t=4 h). The determination of the alcohol component in the anolyte indicated a rise of 2.3% in 20.1 g anolyte. The water component of the solution (<0.2%) remained constant.

Example 6c

[0044] The arrangement of test 4 was used. A voltage difference of 6 V was immediately applied and not changed for 4 hours. The current density dropped from 61.8 A/m² (t=0 h) to 12.5 A/m² (t=4 h). The determination of the alcohol component in the anolyte indicated a rise of 2.5% in 20.5 g anolyte. The water component of the solution (<0.2%) remained constant.

[0045] All references cited herein are fully incorporated by reference. Having now fully described the invention, it will be understood by those of skill in the art that the invention may be performed within a wide and equivalent range of conditions, parameters and the like, without affecting the spirit or scope of the invention or any embodiment thereof 

What is claimed is:
 1. A method for the electrochemical production of alkali alcoholates starting from a saline, aqueous anolyte and an alcoholic catholyte, comprising performing electrolysis in an electrolytic cell in which the cathode compartment and the anode compartment are separated by a solid electrolyte that conducts only ions and which is anolyte- and catholyte-stable.
 2. The method of claim 1, wherein β-aluminum oxide is used as said solid electrolyte.
 3. The method of claim 1, wherein said solid electrolyte is coated on at least one side with one or more ion-permeable, electrically conductive layers.
 4. The method of claim 3, wherein the ion-permeable, electrically conductive layer is applied onto the solid electrolyte by vapor deposition or is burned onto its surface as an immersion layer.
 5. The method of either claim 1 or claim 2, wherein the cathode side and the anode side surfaces of said solid electrolyte are coated with an electrically conductive, porous layer in contact with means supplying and removing current.
 6. The method according to any one of claims 1-4, wherein said solid electrolyte is in the form of a plane-parallel plate.
 7. The method according to any one of claims 1-4, wherein said electrolytic cell comprises an anode compartment and a cathode compartment, at least one of which is filled with an electrically conductive powder.
 8. The method of any one of claims 1-4, wherein the temperature of the anolyte during electrolysis is between −10° C. and 0° C., and the temperature of the catholyte during electrolysis is between −45° C. and −5° C., calculated in each instance from the boiling point of the anolyte or of the catholyte.
 9. The method of any one of claims 1-4, wherein said method is carried out in a continuous manner.
 10. The method of any one of claims 1-4, wherein the anolyte has a pH of greater than
 7. 11. An electrolytic cell comprising: an anode compartment containing an aqueous solution of at least one alkali-metal salt as anolyte; a cathode compartment; and a solid electrolyte as membrane by means of which said anode compartment and said cathode compartment are separated from one another.
 12. The electrolytic cell of claim 11, wherein the solid electrolyte does not comprise any other ion-conducting layer.
 13. The electrolytic cell of claim 11 or claim 12, wherein β-aluminum oxide is used as the solid electrolyte.
 14. The electrolytic cell of claim 11 wherein the solid electrolyte is coated on at least one side with one or more ion-permeable, electrically conductive layers.
 15. The electrolytic cell of claim 14, wherein β-aluminum oxide is used as the solid electrolyte.
 16. The electrolytic cell of either claim 14 or claim 15, wherein the ion-permeable, electrically conductive layer is applied onto the solid electrolyte by vapor deposition or is burned onto its surface as an immersion layer.
 17. The electrolytic cell of claim 11, wherein the cathode side and anode side surfaces of the solid electrolyte are coated with an electrically conductive, porous layer in contact with means supplying and removing current.
 18. The electrolytic cell of any one of claims 11, 12, 14, 15 or 17, wherein the solid electrolyte is in the form of a plane-parallel plate.
 19. The electrolytic cell of any one of claims 11, 12, 14, 15, or 17, wherein the anolyte has a pH of greater than
 7. 20. A method of producing alkali alcoholates, comprising performing electrolysis using the electrolytic cell of any one of claims 11, 12, 14, 15, or
 17. 